1. Field of the Invention
The invention relates to a cooled wall having cooling-air bores, in particular film-cooling bores.
2. Discussion of Background
To increase the output and the efficiency, increasingly higher turbine inlet temperatures are being used in modern gas-turbine plants. In order to protect the turbine blades from the increased hot-gas temperatures, they must be intensively cooled. At correspondingly high inlet temperatures, purely convective cooling is no longer sufficient. The film-cooling method is therefore often used. In this case, the turbine blades are protected from the hot gas by a cooling film. To this end, openings, for example bores, through which the cooling air is blown out, are made in the blades.
In order to achieve as high a cooling effect as possible, the cooling air which is blown out must be deflected as rapidly as possible and flow in a protective manner along the profile surface. In order to also protect the zones lying between the bores, rapid lateral spreading of the cooling air is also necessary. This may be achieved by the cooling-air bores having a diffuser, which on account of the lateral widening permits a wider area of the surface to be covered. To further improve the mixing behavior, geometrical diffuser forms in which the bore is widened not only laterally but also on the downstream side of the bore are used. The blow-out rates in the case of these geometrical diffuser forms are small, so that there is little risk of the cooling air passing through the flow boundary layer. The cooling efficiency can therefore be increased considerably compared with a cylindrical bore.
Publication U.S. Pat. No. 4,197,443 discloses a spark-erosion electrode with which holes which widen in the lateral and longitudinal directions can be made in airfoils.
Publication EP-B-228 338 describes a cooled wall having a cooling-medium passage which has a dosing section and a diffuser section. The diffuser section contains a plane surface in the downstream and upstream directions respectively. Two side surfaces diverge from one another toward the cooling-medium outlet.
Experience shows that malfunctions repeatedly occur during the operation of a gas turbine, and these malfunctions may lead to parts of the machine becoming detached, being transported through the turbine and causing damage in the process. The regions having considerable flow deflection are most affected by the impact of foreign bodies. The foreign bodies generally have a higher specific density than the hot gas flowing through the machine. Consequently, the foreign bodies are deflected to a less pronounced degree at these locations and strike a wall. Typical impact locations are found, for instance, in the region of the leading edge on the suction side of turbine blades. If the foreign bodies strike those locations of the blade at which cooling bores are made, there is the risk in the case of the hitherto known geometrical diffuser forms that the hole cross section will be reduced or even completely closed. As a result, the cooling effect is greatly reduced. With the tight design limits normal nowadays, this may lead to the maximum permissible material temperatures being exceeded, which drastically reduces the service life of the blade.